Various feedback control systems are known and used to clean exhaust gases from automotive internal combustion engines. Such a control system uses an electric signal generated by an oxygen sensor operative to detect and determine the concentration of oxygen in the exhaust gases from an internal combustion engine. The electric signal thus produced by the oxygen sensor is processed with a view to cleaning the exhaust gases from the engine and enables the control system to properly regulate the air-to-fuel ratio of the combustible mixture to be inducted into the combustion chambers of the engine.
Among the various types of oxygen sensors used for such exhaust-gas cleaning purposes, there is an electrochemical oxygen sensor using an oxygen-ion conductive solid electrolyte as an active material. An example of an electrochemical oxygen sensor of this type is disclosed in U.S. Pat. No. 4,450,065. The electrochemical oxygen sensor shown in this published Patent is operative to generate a voltage output which varies in proportion to the detected concentration of oxygen. A feedback control using this oxygen sensor is, for this reason, capable of regulating the air-to-fuel ratio with far more subtleness than could be achieved by means of oxygen sensors of the types which generate voltages not variable in proportion to the detected concentrations of oxygen. Problems are however met in known oxygen sensors with outputs proportional to oxygen concentrations in that such oxygen sensors are much more susceptible to temperature variation than oxygen sensors with outputs non-proportional to oxygen concentrations. When such a proportional-output oxygen sensor is used in an internal combustion engine which is subject to frequent fluctuations in operating temperature, the voltage output from the sensor will also fluctuate frequently. The voltage output from the oxygen sensor will decrease in response to reduction in the temperature of the exhaust gases from the engine and will increase as the power output of the engine is increased to cause pulsation in the flow of exhaust gases to invite steep fluctuations in the gas pressure in the exhaust system in which the oxygen sensor is located. Such an unstable voltage output from the oxygen sensor will require provision of a sophisticated architecture in the signal processing circuitry to process the signal for the control of the air-to-fuel ratio of the combustible mixture to be supplied to the engine.
It is accordingly a prime object of the present invention to provide an improved electrochemical oxygen sensor assembly adapted to produce an electric output which is less dependent upon temperature fluctuations in the atmosphere to be monitored by the sensor assembly than in a conventional oxygen sensor having a proportional concentration-output characteristic.
It is another important object of the present invention to provide an improved oxygen sensor assembly capable of generating a stable, fluctuation-free electric output in temperature varying environments as in the exhaust system of an internal combustion engine.
It is still another important object of the present invention to provide an improved oxygen sensor assembly which is reliable in performance under temperature varying conditions and which will also provide all the advantages that are achievable by any conventional oxygen sensors having proportional concentration-output characteristics.
It is still another important object of the present invention to provide an improved oxygen sensor assembly in which the diffusion of oxygen molecules into the oxygen sensor assembly is not seriously affected by the possible deposit of contaminant particles at any locations of the assembly.
Yet, it is still another important object of the present invention to provide an improved oxygen sensor assembly which is advantageous for use in the exhaust system of an internal combustion engine which normally involves pulsation in the pressure of the exhaust gases therefrom.
It is still another important object of the present invention to provide an improved oxygen sensor assembly in which, when used in the exhaust system of an internal combustion engine, the oxygen sensing unit forming part of the assembly can be reliably protected from being abruptly cooled down as would otherwise be caused by the direct impingement of exhaust gases involving pulsating gas pressures.
It is still another important object of the present invention to provide an improved oxygen sensor assembly in which, when used in the exhaust system of an internal combustion engine, the pulsating pressure of the exhaust gases admitted into the sensor assembly can be dampened out before the gases reach the oxygen sensing unit forming part of the assembly and enables the sensing unit to detect the concentration of oxygen in the gases flowing into the gap in the sensing unit at practically constant, relatively low pressure levels.